Process and apparatus for low-emission storage of biodegradable matter

ABSTRACT

A process and an apparatus for low-emission storage of biodegradable materials by means of aeration, without aerobic conditions materializing in the material, permits a more consistent feed of the downstream stages in the process. Aeration inhibits methanogenesis and the risk potential of an explosive gas mixture forming in the storage vessel or in the components of the system exhausting waste gas. Closed loop control of the aeration adapts the rate of aeration to the biological activity in the stored material and minimizes air input so as to minimize the loss of methanogenesis potential by aerobic conversion of the matter whilst minimizing the energy required for aeration. The process is characterized in that controlling the aeration in the storage vessel inhibits relevant methanogenesis.

TECHNICAL FIELD

The present invention relates to low-emission storage of biodegradablematerials by means of aeration without aerobic conditions materializingin the material, whereby the aeration rate is adapted to the biologicalactivity in the material for storage so that no methane is emitted.

BACKGROUND OF THE ART

Due to their usage or their environmental exposure biodegradablematerials often contain a sufficiently versatile biocenosis ofmicroorganisms which biologically degrade these materials completely orpartly, depending on the environment concerned, meaning thatbiodegradation spontaneously materializes in digestion of thesematerials. This applies especially for wastes having a relevant contentof the biogenic-organic materials containing, simply from their origin,a corresponding concentration of microorganisms.

Where an adequate supply of oxygen exists, the carbon bound in theorganic substance is converted into cell mass and carbon dioxide, theseproducts corresponding to the natural products of metabolism in theearth's atmosphere. If, however, the oxygen supply is insufficient toensure aerobic conditions, anaerobic biodegradable processes occur whichultimately can result in methane being generated. When this happens,methane is emitted in open storage of the matter, a gas which in theambient air poses a relevant danger of explosion and has a highgreenhouse potential. This is why storage vessels holding biologicallyactive materials are covered and means for collecting and treating thewaste gases connected, or in the case of mixed wastes containing waterthe material is anaerobically digested in achieving a controlledmethanogenesis.

It is often the case that biodegradable materials occur discontinuallyor are conditioned batchwise in making use of them biologically. For anoptimum or steady feed of the downstream bioreactors, buffering thematerials is consequently necessary. When subsequently treatedaerobically, aerated storage of materials materializes automatically.But if the material is to be digested anaerobically, storage withexclusion of air is obvious, since aerobic storage uses a lot of energyfor aerating the materials, resulting in a relevant conversion ofpotentially methanogenic substances into carbon dioxide.

When the material is anaerobically stored, the anaerobic biodegradablematerial is subjected to a chain of degrading reactions. Where organicsolids are concerned, this chain involves hydrolysis of the solids,acidification of the dissolved intermediate products (acidogenesis),conversion of the resulting acids into acetic acid, hydrogen and carbondioxide (acetogenesis) ending in the formation of methane(methanogenesis). Responsible for each step in this conversion arecertain groups of microorganisms in each case. When this chain ofinterdependent reactions is balanced, i.e. when the conversion rates ofeach step in the reaction are equal, the products of a partial step arefurther made use of in subsequent steps and there is no accumulation ofintermediate products, as a result of which the biodegradable organiccarbon is converted into methane and carbon dioxide.

However, the various activities involved in the groups of microorganismsmay also result in enrichment of intermediate products, it being mostlythe case that the spontaneous anaerobic degrading of dissolvedbiodegradable substances results in enrichment of organic acids in thesubstrate, since the activity of acidogenic microorganisms issignificantly higher than that of methanogenous substances. When thequantity of enriched organic acids exhausts the buffer capacity, theresult is a drop in the pH which in turn results in a reduction in theactivity of methanogenic microorganisms. The result of this imbalance isan acidified material the low pH of which totally inhibitsmethanogenesis, a typical example for this stabilizing process beingsilage from grass cuttings in agriculture.

This self-inhibition of a completely anaerobic biodegradation ofbiodegradable organic materials comes up against its limits, however,when the biodegradable material in the substrate mix exists mainlyparticulate and insoluble, with a low potential of readily acidifiablecomponents, a high buffer capacity and when the density of methanogenicmicroorganisms is elevated. When this is the case, the resulting organicacids are buffered and there is no significant drop in the pH of thesubstrate mix, resulting in the methanogenic activity being maintainedand hydrolysis being the step determining the rate in anaerobicbiodegradation (Noike et al. (1985): Characteristics of CarbohydrateDegradation and the Rate-Limiting Step in anaerobic Digestion,Biotechnology and Bioengineering 27, pp 1482-1489).

In actual practice such conditions exist, for example, in anaerobicdigestion of biowastes from selective wastes collection. Depending onthe time of year involved these wastes feature a relative low percentageof soluble, readily digestable organic matter but a considerablepercentage of particulate biomass (e.g. garden waste). Furthermore,wastes of this kind are often mashed with process water before digestion(EP 0 520 172, DE 198 33 776, DE 199 07 908). This process water ispreferably obtained from dewatering digested waste, it thus containingboth an elevated density of methanogenic microorganisms and a highbuffer capacity. The buffer capacity materializes in the digestion ofthe methane itself from formation of hydrocarbonates, mainly ammoniumhydrocarbonate. TAC values of 4 to 8 g/l are often found in the processwater. In anaerobic storage of the suspension the result of this is thespontaneous formation of organic acids being too weak to substantiallylower the pH and the methanogenic activity from the process water issufficient to convert organic acids formed as a result of the solidshydrolysis into methane. Methane and carbon dioxide are thus generatedin the storage tank from part of the organic carbon.

When methane is formed in storage of the suspension, connecting thebioreactor to a means for collecting biogas is an obvious processsolution, as disclosed in EP 1 280 738. The drawback in this aspect is,however, that by connecting the storage vessel to the biogas collectionsystem the fluctuations in the quality of the biogas become even morepronounced. The biogas formed in the storage vessel is characterized bya low content of methane and a high content of carbon dioxide due to thepredominant anaerobic reactions in hydroloysis and acidification.Furthermore, the materials supplied to this storage vessel often featuregreatly fluctuating volume flows in brief intervals whilst the storagevessel material is tapped relatively consistently, resulting in heavyfluctuations in the levels in the storage vessel.

When material is supplied to the storage vessel, low-methane biogas isdisplaced from the reactor into the biogas collection system, resultingin addition to the flow of biogas from the digesters a high volume flowof biogas having a low methane content, causing a brief drop of themethane content in the biogas being produced at the time. On completionof the feed to the storage vessel, the level therein drops and a totalcollapse in the flow of biogas from the storage vessel may occur,resulting in a strong increase in the methane content in the biogasprompting corresponding fluctuations in the calorific value. Since thesystems recycling biogas are designed on the basis of the calorificvalue of the biogas, such fluctuations in the calorific value disruptoperation in making use of the biogas. This can only be avoided byinstalling a corresponding large biogas storage capacity which, however,adds to the costs of investment and operation. Furthermore, connectingthe storage vessel to the biogas collection system results in a drop inthe mean methane content in the biogas and thus a deterioration inquality.

For example, digestion of 70 t of biowaste from the separated collectionof domestic waste produces a biogas volume flow of approx. 7,200 m³/ddaily. When distributed over the full day this biogas production resultsin a mean volume flow of 300 m³/h and a methane content of approx. 60%by volume. But conditioning the waste material is done batchwise and theresulting waste suspension is discharged with a volume flow of approx.160 m³/h into the storage vessel. Because of the displacement thisresults in an additional biogas flow of 160 m³/h with a methane contentof approx. 20% by volume. This in turn briefly results in a biogasvolume flow of 460 m³/h with a methane content of approx. 46% by volume,in other words, the calorific value of the biogas drops briefly byalmost 25%.

DE 198 33 776 shows the necessity of providing a storage vessel upstreamof the digestor but with no indication of how to avoid gas emissionsfrom the storage vessel. Although in EP 1 280 738 connecting the storagevessel to the biogas or digestion gas collection system is described,making such a connection results in trouble in operation, in the absenceof a sufficient digestion gas storage volume when making use of thedigestion gas, due to the fluctuations in the calorific value.

SUMMARY OF THE INVENTION

Object of the invention is low-emission storage of biodegradablematerials, now making it possible to feed downstream steps in theprocess more consistently. Methanogenesis is inhibited by aeration inthus reducing the risk of an explosive gas mixture forming in thestorage vessel or in system waste gas components. Closed loop control ofaeration adapts the rate thereof to the biological activity in thestored material so as to minimize inclusion of air in thus minimizingboth the loss in methanogenic potential due to aerobic matter conversionand in the energy needed for aeration.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be detained by describing embodiments of theprocess and example aspects of the apparatus with reference to thedrawing in which:

FIG. 1: is a block diagram of how the process in accordance with theinvention is managed

FIG. 2: is a block diagram of process control in accordance with theinvention showing how the rate of aeration is controlled

FIG. 3: is a block diagram corresponding to that as shown in FIG. 2 butrelating to an alternative embodiment for controlling the rate ofaeration

FIG. 4: is a block diagram corresponding to that as shown in FIG. 2 butrelating to another alternative embodiment for controlling the rate ofaeration

FIG. 5: is a block diagram of an example of a process controller inmanaging the process in accordance with the invention

FIG. 6: is a block diagram showing a simplified process controller

FIG. 7: is an illustration of a preferred aspect

FIG. 8: is an illustration of a further preferred aspect.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

Referring now to FIG. 1 there is illustrated how the biodegradablematerial (11) is made available in a supply zone for treatment in thedownstream stages of the process. It is here that any conditioning ofthe material may be done to ensure smooth operation of the downstreamstages of the process. The furnished or also conditioned material (21)is supplied to the storage vessel (2) where it is buffered. Charging thestorage vessel (2) with material from the supply or conditioning zone isdictated exclusively by the requirements of the supply or conditioningzone. From the storage vessel (2) the charging (31) of the reactor foranaerobic digestion (3) is activated, discharge of the biodegradablematerial from the storage vessel (2) being dictated exclusively by therequirements of the digestion stage (3). The storage vessel (2) receivesa supply of ambient air (22) to avoid anaerobic storage of thebiologically active materials, although instead of ambient air (22) aflow of compressed air or compressed oxygen can be supplied. The air orgases (23) formed by the biological activity in excess are expelled fromthe storage vessel (2). The waste gas (41) from the storage vessel (2)is supplied to a waste gas scrubber (4) and after cleaning, emitted asclean gas (42). When the waste gas (41) contains no odorous or noxiouscomponents it can also be discharged directly.

In a preferred aspect of the process, part of the waste gas (24) fromthe storage vessel (2) is returned to the storage vessel (2). Returningthe partial flow (24) results in a better distribution of the suppliedair (22) and an improvement in the transition of the matter by mixingthe content of the vessel.

Adapating aeration can be done on the basis of detecting the methane inthe waste gas or directly from the gas phase in the vessel eithermanually or in online-methane detection or by a control algorithm. Indetecting the methane in the waste gas or in the gas phase of thestorage vessel the aeration is increased incrementally as a function ofthe increase in the methane concentration. When detecting the methane iszero the rate of aeration is slowly reduced in increments.

Referring now to FIG. 2 there is illustrated a preferred aspect of theprocess showing how the rate of aeration is controlled. The storagevessel (2) receives a supply of biodegradable material (21) inaccordance with the requirements of the upstream stages in the process.Furthermore, the content of the storage vessel (2) is fed to thedownstream stages of the process. A gas (24) is taken from the gas phaseof the storage vessel (2) by means of a fan (241) and is returned evenlydistributed in the bottom region of the storage vessel (2) by means ofthe distribution system (242). A further fan (delivery means (221))feeds air to the storage vessel (2) by the distribution system (242) toavoid anaerobic conditions in the vessel.

In the gas discharge conduit (23) of the storage vessel (2) or in thegas recirculating conduit (24) the methane content of the gas phase ofthe storage vessel (2) is detected by means of a methane analyzer (5).On the basis of the value detected by the methane analyzer (5) the airdelivery of the delivery means (221) is controlled. When the gasanalyzer (5) indicates a methane content the delivery of the deliverymeans (221) is increased until the methane detected has returned tozero. When the reading of the gas analyzer (5) is zero for apredetermined interval, the delivery of the delivery means (221) isgradually reduced until a methane reading is again possible.Subsequently, the delivery of the delivery means (221) is reincreased bya fixed amount. For closed loop control of air delivery by the deliverymeans (221) as a function of the methane detection in the gas phase aPID or fuzzy controller can be employed.

Referring now to FIG. 3 there is illustrated a block diagram showing howsetting the aeration can also be done, using the Redox potential as thecommand variable. By means of a delivery means (241) a partial flow issampled from the storage vessel (2) and supplied to a Redox analyzer(6). The discharge (25) of the Redox analyzer (6) is returned to thestorage vessel (2).

When the Redox potential drops below a preset value the delivery of thedelivery means (221) is increased incrementally. When the Redoxpotential exceeds a preset value the delivery of delivery means (221) isdecreased incrementally. In this arrangement, the preset value for theRedox potential is established from values gained from experience forthe biodegradable substrate and process controller in each case.Establishing these values can be obtained from lab tests or during thecommissioning phase of the technical system. Experience hitherto showsthat for an effective control of the aeration the Redox potential shouldbe in the preset value range of −220 to 0 mV.

Referring now to FIG. 4 there is illustrated how for controlling theaeration even more precisely the methane content can be detected in thegas phase in combination with detecting the Redox potential of thestorage vessel content. In this case, the air delivery of the deliverymeans (221) is controlled as a function of the deviation of the Redoxpotential from the preset value as a function of the increase in themethane content. When the analysis with both instruments indicates thepreset value being exceeded, the delivery of the delivery means (221) isincreased further. Increasing the delivery of delivery means (221) isadapted on the basis of the change in the Redox potential and methanecontent. When the Redox potential exceeds the preset value and nomethane is detected in the gas phase of the storage vessel, the deliveryof delivery means (221) is reduced incrementally as a function of thechange in the Redox potential.

The results of tests obtained from the storage of pulps of organicwastes show how effective aeration is in reducing formation of thegreenhouse gas methane. In these tests an aerated and a non-aeratedstorage vessel were operated in parallel and the available methanecontent determined in the waste gas during the storage duration of theindividual batches. The results of these tests are shown in thefollowing Table, making it clear that as of an adequate aeration themethane content detected in the waste gas had dropped to zero.

Aerated storage vessel Non-aerated storage vessel Waste gas CH₄ in Wastegas CH₄ in Aeration rate flow waste gas Aeration rate flow waste gas[l/(h × kg DM)] [l] [vol %] [l/(h × kg DM)] [l] [vol %] 1.7 17.4 5.0 011.7 18.4 3.0 15.5 2.5 0 13.7 12.6 6.3 18.2 0.0 0 15.6 Unknown 8.1 15.90.0 0 11.1 11.1 DM = dry mass of storage material

The Redox potential values obtained in the aerated storage vesselindicated a correlation between methanogenesis and Redox voltage.Increasing the Redox voltage results in a reduction in themethanogenesis rate. At Redox voltages exceeding approx. −100 mV methanegas production in testing was zero.

Referring now to FIG. 5 there is illustrated an example of an algorithmfor controlling a process as managed in accordance with the invention.For controlling the delivery of the delivery means (221) the results ofa gas analyzer for determining the methane content in the waste gas ofthe storage vessel and of a means for determining the Redox potential inthe stored material are available. The control algorithm is structuredas follows:

-   -   1. If methane is detected in the waste gas of the storage        vessel, the delivery of the delivery means (221) is increased by        a predetermined percentage. This percentage is defined as the        product of the detected methane value and a constant K1. After a        predetermined waiting period the actual/preset value comparison        is repeated.    -   2. If no methane is detected in the waste gas of the storage        vessel the Redox potential is sensed. If this is below −100 mV        there is a risk of methanogenesis. This is why the delivery of        the delivery means (221) is increased by a predetermined        percentage K2 and after a predetermined waiting period the        actual/preset value comparison is repeated.    -   3. If the Redox potential exceeds −20 mV the aeration rate of        the stored material is unnecessarily high and the delivery of        the delivery means (221) can be reduced by a predetermined        percentage K3. After a predetermined waiting period the        actual/preset value comparison is repeated.    -   4. If no methane is detected in the waste gas of the storage        vessel and the Redox potential of the stored material is in the        range −100 and −20 mV, the actual/preset value comparison is        repeated.

Referring now to FIG. 6 there is illustrated the algorithm of asimplified controller based exclusively on detecting methane in thewaste gas of the storage vessel.

-   1. When methane is detected in the waste gas of the storage vessel,    then:    -   I. the delivery of the fan (221) is increased by a predetermined        percentage. This percentage is defined as the product of the        detected methane value and a constant K1. On timeout of a        predetermined waiting period the actual/preset value comparison        is repeated.    -   II. The timer T is reset to zero.        On timeout of a predetermined waiting period the actual/preset        value comparison is repeated.-   2. If no methane is detected in the waste gas of the storage vessel    the time T1 of the timer T is checked. If T1 is greater than the    critical time T2, the aeration of the stored material is    unnecessarily high and    -   I. the delivery of the fan (221) is reduced by a predetermined        percentage K3 and    -   II. the timer T is reset to zero.    -   On timeout of a predetermined waiting period the actual/preset        value comparison is repeated.-   3. If no methane is detected in the waste gas of the storage vessel    and if the T1 is smaller than the critical time T2 the actual/preset    value comparison is repeated.

EXAMPLE EMBODIMENTS Example 1

Referring now to FIG. 7 there is illustrated a preferred embodiment ofthe apparatus in accordance with the invention for pumping the inflow(21) and outflow (31 a) of the material is pumped when the storagematerial is pumpable. For this purpose centrifugal or displacement pumps(211; 311) can be employed depending on the integration hydraulically.As an alternative, the outflow may also be activated by means of a finalcontrol element (312) in gravity flow (31 b). To circulate the contentof the storage vessel (2) a partial flow is sampled from the exhaustflow (23) and entered into the stored material by means of a compressor(241; e.g. positive displacement compressor) via a lance system (242)located centrally above the middle of the floor of the vessel. Therising gas bubbles create a strong loop flow (243) ensuring a thoroughintermixing of the vessel contents. A lance system with its top feed hasthe advantage that when the lance system is in need of repair, thevessel does not need to be emptied, but instead, the lance systemremoved from the top of the vessel. At the suction end of the compressor(241) a blower (221) e.g. fan) transports the required air 22 into thecirculating gas flow. A further blower (511) e.g. fan) samples from theexhaust air flow (23) a further partial flow and feeds it to a methaneanalyzer (5; e.g. infrared absorption analyzer).

Referring now to FIG. 8 there is illustrated a preferred embodiment ofthe apparatus in accordance with the invention showing how a screwconveyor (211; 311) is used to handle inflow (21) and outflow (31) ofthe material. For aerating the content of the storage vessel (2) apartial flow is sampled from the exhaust flow (23) and returned to thevessel by means of a delivery assembly (241: e.g. rotary spool valve).Aeration occurs via a slotted or perforated tray (242) at the bottom ofthe storage vessel (2) over which the discharge screws run, the flowthereof being crosswise to the slotted tray to prevent the latter frombecoming clogged up. Provided below the slotted tray in the vessel is amaintenance port (244) permitting maintenance of the slotted tray aswell as removal of material having fallen therethrough. Provided at thepressure end of the delivery assembly (241) is a nozzle or aperture(245) via the vacuum zone of which ambient air (22) can be aspirated. Bymeans of a final control element (221) in the air intake conduit theincoming air flow can be varied. A blower (511; e.g. fan) samples fromthe exhaust air flow (23) a further partial flow and feeds it to amethane analyzer (5 e.g. thermal conductivity analyzer).

Advantages:

The process in accordance with the invention permits cost-effective,low-emission storage of biodegradable materials. Inhibitingmethanogenesis makes for the following improvements:

The storage vessel now permits a more consistent charging of thedownstream stages in the process which in turn achieves longer runningperiods of the apparatus and smaller thruputs as well as a moreconsistent production of biogas. Furthermore, the storage vessel nolonger needs to be connected to the biogas collection of a downstreamdigester in thus significantly reducing fluctuations of the methanecontent in the biogas whilst increasing the mean methane content. Bothof these factors enhance the efficiency of a process stage for recyclingbiogas whilst minimizing the biogas storage volume required with all theeconomic advantages of: lower costs for biogas storage and recycling,since its production is now more consistent, together with higherefficiency in recycling the biogas since its quality is more consistent.

Closed loop control of the aeration minimizes the air intake in thusminimizing the loss of methanogenesis potential by conversion of theaerobic matter as well as the energy required for aeration in ensuringmaximum energy yield from a downstream stage for anaerobic digestion.

The risk potential of an explosive gas mixture forming in the storagevessel or in the exhaust components of the system is now reduced,resulting in a reduction in the costs for system safeguards.

Since the storage vessel can now be decoupled from the biogas collectionsystem the components handling biogas in the system and thus theexplosion protection zones are reduced.

The invention claimed is:
 1. A method of processing biodegradablematerial, comprising: supplying the biodegradable material to a storagevessel for buffering therein, removing waste gas generated in thestorage vessel, mixing at least some of the removed waste gas with anoxygen-containing gas, aerating the biodegradable material in thestorage vessel by supplying at least some of the mixture of waste gasand oxygen-containing gas to the storage vessel, whereby biologicalactivity of the biodegradable material is modified in a manner such asto inhibit at least one of methanogenesis and formation of an explosivegas mixture in the storage vessel, and supplying the biodegradablematerial from the storage vessel to a biogas reactor for anaerobicdigestion to generate biogas by means of hydrolysis, acetogenesis andmethanogenesis.
 2. The process as set forth in claim 1, wherein themixing step comprises mixing waste gas with ambient air.
 3. The processas set forth in claim 1, comprising detecting methane content in thewaste gas removed from the storage vessel or in the waste gas returnedto the storage vessel and activating a closed loop control of aerationof the storage vessel depending on detection of methane content.
 4. Theprocess as set forth in claim 3, comprising detecting the methanecontent by means of infrared absorption or thermal conductivity.
 5. Theprocess as set forth in claim 1, comprising sensing the Redox potentialof biodegradable material in the storage vessel and activating a closedloop control of aeration of the storage vessel depending on the sensedRedox potential.
 6. The process as set forth in claim 5, comprisingmaintaining the Redox potential of the storage medium in the range −220to 0 mV.
 7. The process as set forth in claim 1, comprising detectingmethane content in the waste gas removed from the storage vessel or inthe waste gas returned to the storage vessel, sensing the Redoxpotential of biodegradable material in the storage vessel, andactivating a closed loop control of aeration of the storage vesseldepending on detection of methane content and on the sensed Redoxpotential.
 8. A method of processing biodegradable material, comprising:supplying the biodegradable material to a conditioning zone,conditioning the biodegradable material in the conditioning zone toprepare the biodegradable material for anaerobic digestion, supplyingthe biodegradable material from the conditioning zone to a storagevessel for buffering therein, removing waste gas generated in thestorage vessel, mixing at least some of the removed waste gas with anoxygen-containing gas; aerating the biodegradable material in thestorage vessel by supplying at least some of the mixture of waste gasand oxygen-containing gas to the storage vessel, whereby biologicalactivity of the biodegradable material is modified in a manner such asto inhibit at least one of methanogenesis and formation of an explosivegas mixture in the storage vessel, and supplying the biodegradablematerial from the storage vessel to a biogas reactor for anaerobicdigestion to generate biogas by means of hydrolysis, acetogenesis andmethanogenesis, wherein the biodegradable material is supplied to thestorage vessel exclusively in accordance with requirements of theconditioning zone and the biodegradable material is supplied to thebiogas reactor exclusively in accordance with requirements of the biogasreactor.
 9. An apparatus for implementing the method according to claim1, comprising: a biogas reactor for anaerobic digestion of biodegradablematerial to generate biogas by means of hydrolysis, acetogenesis andmethanogenesis, a storage vessel upstream of the bioreactor forbuffering biodegradable material before the biodegradable material issupplied to the biogas reactor, the storage vessel having a waste gasoutlet and an aerating gas inlet, and ducting connected to the storagevessel and to a source of oxygen-containing gas for removing waste gasgenerated in the storage vessel via the waste gas outlet, mixing atleast some of the removed waste gas with oxygen-containing gas from saidsource, and supplying at least some of the mixture of waste gas andoxygen-containing gas to the storage vessel via the aerating gas inletfor aerating the biodegradable material in the storage vessel.
 10. Theapparatus as set forth in claim 9, wherein the source ofoxygen-containing gas comprises a compressed air delivery system. 11.The apparatus as set forth in claim 9, wherein the source ofoxygen-containing gas comprises a source of compressed oxygen.
 12. Theapparatus as set forth in claim 9, comprising a gas distributionstructure for distributing the oxygen-containing gas flow in thebiodegradable material in the storage vessel.
 13. The apparatus as setforth in claim 9, comprising a duct for diverting a sample of the wastegas removed from the storage vessel and a methane analyzer connected tosaid duct for receiving said sample.